Despite decades of effort, cancer remains an especially difficult disease for development of therapeutics. According to the Cancer Prevention Coalition (University of Illinois), cancer rates have increased 24% in the past thirty years even after adjusting for aging of the population. Remarkably, despite significant progress during this period the overall five-year survival rates have remained virtually static (approximately 50% depending on the cancer). Thus new drugs are required to develop more effective life-saving cancer therapies.
Celecoxib, a selective COX-2 inhibitor, is one of the world's most successful drugs, alleviating pain and inflammation for millions of patients. In addition, COX-2 over-expression has been found in several types of human cancers such as colon, breast, lung, prostate and pancreas, and appears to control many cellular processes. COX-2 plays a role in carcinogenesis, apoptosis and angiogenesis, and therefore represents an excellent drug target for the development of novel medicines for prevention and/or treatment of human cancers. Currently, celecoxib is approved for limited use in the reduction of polyps in familial adenomatous polyposis (FAP).
The Adenoma Prevention with Celecoxib (APC) trial demonstrated human efficacy of celecoxib in the prevention of sporadic colorectal adenoma. However, this trial also showed that the elevated dose of celecoxib required for anti-cancer efficacy was accompanied by concomitant increase in adverse cardiovascular (CV) events (Bertagnolli, et al., Cancer Prev. Res. (Phila) 2, 310-321 (2009)).
Development of more potent or selective COX-2 inhibitors does not improve CV safety; this liability is thought to be a mechanism-based effect. This was demonstrated in the VIGOR trial by Vioxx®, an extremely potent and highly selective COX-2 inhibitor withdrawn from the market in 2004 due to CV concerns about increased risk of heart attack and stroke with long term, high dose use. These facts have undermined the development of novel COX-2 inhibitors and slowed research to expand their utility to other disease indications, such as cancer.
Nitric oxide (NO) is an important endogenous signaling molecule and vasodilator. NO is synthesized from L-arginine by the enzyme NO synthase (NOS), which exists in three distinct isoforms, namely the constitutively expressed endothelial (eNOS) and neuronal (nNOS) forms, and the mainly inducible form (iNOS). Arginine administration has been shown to reduce blood pressure and renal vascular resistance in essential hypertensive patients with normal or insufficient renal function (Am. J. Hypertens. 12, 8-15 (1999)). It has also been shown that NO deficiency promotes vascular side-effects of celecoxib and other COX inhibitors (Blood 108, 4059-4062 (2006)).
The role of NO in cancer is complex; however, pharmacological evidence using NO-releasing compounds of NSAID's has shown increased anti-tumor efficacy in cell culture and animal cancer models. The different molecular mechanisms of NO are expected to simultaneously enhance anti-cancer efficacy of celecoxib and improve CV safety by preventing an increase in blood pressure associated with COX-2 inhibition, while maintaining gastric-sparing properties superior to NSAID's.
Diverse molecular mechanisms of NO delivery are well known. For example, Kashfi, et al. reported that nitric oxide-donating nonsteroidal anti-inflammatory drugs (NO-sulindac, NO-ibuprofen, NO-indomethacin and NO-aspirin) inhibit the growth of various cultured human cancer cells; providing evidence of a tissue type-independent effect (J. Pharmacol. Exp. Ther. 303, 1273-1282 (2002)).
In another example, Ouyang, et al. reported that nitric oxide-donating aspirin prevented pancreatic cancer in a hamster tumor model (Cancer Res. 66, 4503-4511 (2006)).
COX inhibition and cancer: Two isoforms of cyclooxygenase (COX) are known to exist, a constitutive form (COX-1) present in nearly all tissues and an inducible form (COX-2) upregulated in response to inflammatory stimuli. The discovery of COX-2 led to the development of selective COX-2 inhibitors as anti-inflammatory drugs (coxibs), which were shown to be largely devoid of antiplatelet activity and gastrointestinal ulcerogenicity believed to be associated with inhibition of COX-1.
NSAID's are among the most widely used treatments for pain, fever and inflammation, and have long been known to reduce the risk of cancer in multiple organ sites. The use of aspirin in treatment and prevention of cancer has wide-spread support in the medical community; however, the risks of regular aspirin use are also well established and the risk-benefit profile is not sufficient to recommend aspirin treatment for cancer prevention. With the advent of coxibs, research has focused on COX-2 as a target for the treatment and prevention of certain cancers. Compelling data from the APC trial, described above, demonstrated that celecoxib was useful in preventing sporadic colorectal adenoma in patients at high risk for colorectal cancer.
The role of COX-2 in lung cancer: Lung cancer is the leading cause of cancer-related deaths in the US and is responsible for more deaths than breast, prostate and colon cancers combined. Current research suggests that COX-2 and epidermal growth factor receptor, (EGFR), are important mediators in non-small cell lung cancer (NSCLC). In human NSCLC patients, a combination of erlotinib (a tyrosine kinase inhibitor), and celecoxib showed high response rates and demonstrable clinical benefit. NSCLC currently represents one of the preferred indications for COX-2 inhibition cancer therapy.
Mechanism of COX-2 in cancer: A key feature of COX-2 biology is its ability alone to cause cancer formation in a number of transgenic mouse models. COX-2 derived PGE2 plays a prominent role in tumor growth and the most abundant prostanoid in many human malignancies. Metabolism of arachidonic acid by COX-2 leads to the formation of several prostaglandins (PGs) that bind to tumor suppressor p53, preventing p53-mediated apoptosis. COX-2 derived PGE2 promotes epithelial-to-mesenchymal transition and thus increases resistance to EGFR tyrosine kinase inhibitors in lung cancer.
Nitric oxide and cardiovascular safety: Nitric oxide exhibits a number of important pharmacological actions including vascular relaxation (vasodilatation) and inhibition of platelet aggregation and adhesion. Inhibition of NO synthesis leads to an increase in systemic blood pressure. NO also prevents atherogenesis by inhibiting vascular smooth muscle cell proliferation and preventing low-density lipoprotein oxidation and macrophage activation. Vascular NO generation is important in controlling blood pressure, and a growing body of evidence indicates that NO signaling is a key factor in counteracting the onset and development of several CV diseases including hypertension, myocardial infarction and stroke. NO can be used to counteract CV liabilities associated with COX-2 inhibition.
Nitric oxide and gastric sparing properties: NO-releasing COX inhibitors were originally created to improve gastrointestinal (GI) tolerability (Inflammopharmacology 11(4), 415-22 (2003)). Naproxcinod is a NO-releasing pro-drug of the NSAID naproxen. Naproxcinod showed significantly improved GI tolerability compared to naproxen alone in a chronic rat study (Life Sciences 62, 235-240 (1998)). In another example, L-arginine, coadministered with the NSAID ibuprofen, showed a protective effect on gastric mucosa against ibuprofen-induced mucosal lesions (Free Radic. Res. 38(9), 903-11 (2004)).
Nitric oxide and cancer: NO modulates the activity of transcription factor NF-κB, which represents a potential mechanism for inflammation control, but also regulation of apoptotic mechanisms. NO promotes apoptosis and can reverse tumor cell resistance to chemotherapeutic agents. Studies with NO-releasing NSAID's have shown that NO contributes to anti-cancer activity in cell culture and enhanced in vivo efficacy in rodent cancer models. For example, Steele, et al. reported the chemopreventive efficacy of nitric oxide-naproxen in rodent models of colon, urinary bladder and mammary cancers (Cancer Prev. Res. (Phila) 2, 951-956 (2009)).
Nitrate prodrugs useful in the treatment of inflammatory, ischemic, degenerative and proliferative diseases are reported in EP 01336602 (published 20 Aug. 2003). The compounds described therein include celecoxib coupled with one of the following: nitrooxyalkylenecarbonyl; methylimine-aminomethylene-phenylenemethylene-amide-ethylenecarbonyl; or fluoromethylimine-aminomethylene-phenylenemethylene-amide-ethylenecarbonyl to sulfonamide nitrogen yielding, respectively, Structures (1)-(3) below:
